The present invention pertains to medical nuclear magnetic resonance scanners.
Since the invention of the medical nuclear magnetic resonance (NMR) scanning technique by Dr. Raymond Damadian, as set forth in U.S. Pat. No. 3,789,832, this technique has been widely adopted in the medical arts. Medical NMR scanning requires creation of a substantial constant "primary" magnetic field passing through the patient's body. An additional "gradient" magnetic field varying with time typically is superimposed on the primary field. The patient is exposed to radio frequency electromagnetic waves which also vary with time in particular patterns. Under the influence of the magnetic fields and the radio waves, certain atomic nuclei within the patient's tissues resonate and emit further radio waves. By mathematical techniques involving correlation of the magnetic field patterns in use at various times with the radio frequency waves emitted, it is possible to determine the amounts of particular atomic nuclei, and hence the amounts of particular substances, at various locations within the patient's body. This information typically is displayed as an image with shadings corresponding to the concentrations of certain nuclei of interest. The concentrations of different substances ordinarily differ for differing kinds of tissues. Thus, the image created through NMR techniques permits the physician to see organs and soft tissues within the body, and also permits the physician to see abnormalities, such as tumors, within the body. Accordingly, NMR scanning and imaging techniques are being adopted rapidly by physicians.
Medical NMR scanning imposes certain challenging technical requirements for the apparatus. The primary magnetic field must be a strong field, typically on the order of about 1 kilogauss or more and often more than about 10 kilogauss (one Tesla), far stronger than the magnetic fields associated with common magnets. Moreover, the primary magnetic field must be precisely configured. Thus, the primary field, before application of the gradient fields, should be uniform to at least about one part in 1,000 and preferably at least about one part in 10,000 or better in order to provide a useful image. Even better uniformity is more desirable. This strong, uniform primary magnetic field must be maintained over a region of the patient's body large enough to provide medically useful information, typically over a scanning region encompassing a major portion of a cross section through the patient's torso. Further, the magnetic field apparatus typically must be arranged to receive the patient's body, and hence must provide openings large enough for the patient's body to fit within the apparatus. All of these requirements, taken together, pose a formidable technical problem.
Two distinct and fundamentally different approaches to these requirements are currently employed in construction of medical NMR scanners. As set forth in co-pending, commonly assigned U.S. patent application Ser. No. 841,897, filed Mar. 20, 1986 now U.S. Pat. No. 4,675,609, magnetic field producing means such as permanent magnets can be combined with a ferromagnetic metal frame and other components to form a magnetic assembly which provides the primary field. The disclosure of said U.S. Pat. No. 4,675,609 is hereby incorporated by reference herein. Medical NMR scanners incorporating magnetic assemblies according to the U.S. Pat. No. 4,675,609 application have excellent primary fields and hence offer good scanning capabilities. However, it is difficult and expensive to provide particularly strong primary fields in excess of about 1 Tesla, as desired in some NMR scanning techniques, while still meeting all of the other requirements if only permanent magnets are employed.
The other approach has been to employ electromagnets, most notably, complex solenoidal superconducting electromagnets. The windings of a superconducting magnet lose resistance to flow of electric current when cooled below a particular threshold temperature close to absolute zero (-273.degree. C.). Thus superconducting magnets can carry large currents and can create high fields. Superconducting magnet scanners typically employ a pair of loop-like main superconducting coils and a plurality of auxiliary loop-like superconducting coils. The coils are aligned on a common coil axis to form a single large complex solenoid. This solenoid is enclosed in a toroidal or doughnut-shaped vessel or cryostat having a central opening of sufficient size to receive a patient with the patient's body extending generally along the axis of the toroid. Thus, the long axis of the patient's body is aligned with the axis of the solenoid. When the coils are energized, they generate a magnetic field extending through the patient's body, generally along the long axis of the patient's body. Minor amounts of iron or other ferromagnetic materials are occasionally employed for adjusting or "shimming" the shape of the field, but these devices are basically large, air-core solenoids. The shape of the magnetic field in the scanning region is predominantly influenced by the placement of multiple coils. The auxiliary coils utilized in the solenoid are essential to creation of a uniform field with parallel lines of flux extending through the patient's body.
Although substantial fields can be created by means of these devices, they pose significant problems. It is difficult to maintain a uniform, constant primary field, in part because the coils tend to move when energized. Thus, the coils, when energized, exert substantial magnetic forces, on the order of tens to hundreds of tons, pulling the coils towards one another and tending to spread each coil radially outwardly, away from the coil axis. These movements tend to destroy the uniformity of the field. Attempts to brace and support the coils so as to resist these forces introduce additional problems. Typically, the bracing elements used to resist these magnetic forces are entirely enclosed within the cryostat. The bracing elements thus add to the mass which must be supported within the cryostat and add to the volume of material which must be maintained at ultralow temperatures within the cryostat. Thus, these bracing elements complicate design of the system. Even with substantial braces, coil movement still poses significant problems. As the magnetic forces on the coils increase with the square of the field strength, the problems associated with these forces have posed serious impediments to successful medical NMR scanning at very high fields, above about 2 Tesla.
The magnetic field created by an air-core superconductive solenoid extends outside of the scanner. Therefore, any moving magnetic material in proximity to the scanner with disturb the field and impair the image. The substantial magnetic fields extending outside an air-core superconducting solenoid type NMR scanner can be dangerous. When such a device is energized, magnetic material lying loose within a few feet of the device can be attracted with such force that it becomes a deadly missile. These effects can and have caused injuries in operation of air-core superconducting NMR scanners. To minimize the safety hazards and field disturbances, superconducting NMR scanners heretofore have been encased in large and elaborate sanctuaries. In some cases, ferromagnetic shields have been placed around air-core superconducting solenoid NMR scanners. Ordinarily, these shields have been disposed at considerable distances from the coils and from the patient receiving portions of the device, so that these shields exert no appreciable influence on the shape of field in the scanning region.
Moreover, at least some of the coils in an air-core superconducting scanner typically must be disposed in close proximity to the scanning region. Therefore, the walls of the cryostat enclosing the coils must closely surround the scanning region. As the gradient coils also are disposed close to the scanning region, the gradient coils typically must be disposed adjacent the cryostat walls. It is typically desirable in NMR scanning to vary the fields imposed by the gradient coils at relatively rapid rates. In some cases, non-metallic, electrically non-conductive materials have been utilized instead of metals for the walls of the cryostats to avoid creation of eddy currents in the cryostat wall as the field imposed by the gradient coils varies. These materials are less desirable than metals with regard to strength and other properties desired in the cryostat wall.
For all of these reasons, there have been significant needs heretofore for further improvement in medical NMR scanners.